[PUSHED] gdb: fix libstdc++ assert caused by invalid use of std::clamp
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Commit Message
After this commit:
commit 33ae45434d0ab1f7de365b9140ad4e4ffc34b8a2
Date: Mon Dec 4 14:23:17 2023 +0000
gdb: Enable early init of thread pool size
I am now seeing this assert from libstdc++:
/usr/include/c++/9/bits/stl_algo.h:3715: constexpr const _Tp& std::clamp(const _Tp&, const _Tp&, const _Tp&) [with _Tp = int]: Assertion '!(__hi < __lo)' failed.
This may only be visible because I compile with:
-D_GLIBCXX_DEBUG=1 -D_GLIBCXX_DEBUG_PEDANTIC=1
but I haven't checked. The issue the assert is highlighting is real,
and is caused by this block of code:
if (n_threads < 0)
{
const int hardware_threads = std::thread::hardware_concurrency ();
/* Testing in #29959 indicates that parallel efficiency drops between
n_threads=5 to 8. Therefore, clamp the default value to 8 to avoid an
excessive number of threads in the pool on many-core systems. */
const int throttle = 8;
n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
}
The arguments to std::clamp are VALUE, LOW, HIGH, but in the above, if
we have more than 8 hardware threads available the LOW will be greater
than the HIGH, which is triggering the assert I see above.
I believe std::clamp is the wrong tool to use here. Instead std::min
would be a better choice; we want the smaller value of
HARDWARE_THREADS or THROTTLE. If h/w threads is 2, then we want 2,
but if h/w threads is 16 we want 8, this is what std::min gives us.
After this commit, I no longer see the assert.
---
gdb/maint.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
base-commit: b17ef9dcd8d16eedf4e60565cd7701698b5a0b6b
@@ -855,11 +855,12 @@ update_thread_pool_size ()
if (n_threads < 0)
{
const int hardware_threads = std::thread::hardware_concurrency ();
- /* Testing in #29959 indicates that parallel efficiency drops between
- n_threads=5 to 8. Therefore, clamp the default value to 8 to avoid an
- excessive number of threads in the pool on many-core systems. */
- const int throttle = 8;
- n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
+ /* Testing in PR gdb/29959 indicates that parallel efficiency drops
+ between n_threads=5 to 8. Therefore, use no more than 8 threads
+ to avoid an excessive number of threads in the pool on many-core
+ systems. */
+ const int max_thread_count = 8;
+ n_threads = std::min (hardware_threads, max_thread_count);
}
gdb::thread_pool::g_thread_pool->set_thread_count (n_threads);